gradethis

Overview

gradethis helps instructors provide automated feedback for interactive exercises in learnr tutorials. If you are new to writing learnr tutorials, we recommend that you get comfortable with the learnr tutorial format before incorporating gradethis.

gradethis provides two grading modalities. You can:

1. Compare student code to model solution code with grade_this_code(), or

2. Write custom grading logic with grade_this().

To use gradethis in a learnr tutorial, load gradethis after learnr in the setup chunk of your tutorial:

```{r setup}
library(learnr)
library(gradethis)
```

To help authors provide consistent feedback, gradethis uses global options to control the default behavior of many of its functions. You can set or change these default values using gradethis_setup().

Checking Student Code

Suppose we ask our students to calculate the average of the first ten integers. In our tutorial’s R Markdown, we include a prompt, followed by an exercise chunk.

**Calculate the average of all of the integers from 1 to 10.**

```{r average, exercise = TRUE}
____(1:10)
```

Our hope is that students will replace the ____ with the correct R function, in this case mean().

To grade an exercise, we need to associate checking code with the exercise. In learnr, this code is written in a chunk named <exercise label>-check, where <exercise label> is the label of the chunk with exercise = TRUE.

To use gradethis to grade our average exercise, we create a new chunk named average-check and we call either grade_this() or grade_this_code() inside the chunk.

Custom Grading Logic

In the average-check chunk, we can either call grade_this() or grade_this_code(). The first gives authors control over the grading logic, which is typically written inside curly braces as the first argument of grade_this():

**Calculate the average of all of the integers from 1 to 10.**

```{r average, exercise = TRUE}
____(1:10)
```

```{r average-check}
grade_this({
  # custom grading code
})
```

We’ll cover custom grading logic in more detail below.

Compare with Solution Code

On the other hand, grade_this_code() requires authors to include a model solution to which the student’s submission will be compared. The solution is written in the <exercise label>-solution chunk, while grade_this_code() is called in the <exercise label>-check chunk:

**Calculate the average of all of the integers from 1 to 10.**

```{r average, exercise = TRUE}
____(1:10)
```

```{r average-solution}
mean(1:10)
```

```{r average-check}
grade_this_code()
```

When a student submits their exercise solution, it is automatically compared to the model solution in the -solution chunk, and the first encountered difference is reported to the student. If there are no differences, a positive statement is returned to the student.

Beautiful! Correct!

If the student submits an incorrect answer, such as max(1:10), the feedback message attempts to point the student in the right direction.

I expected you to call mean() where you called max(). Try it again. I have a good feeling about this.

Writing Custom Logic

If you want to provide custom feedback for specific errors that students may be likely to make in your exercise, grade_this() provides a high level of flexibility.

When using grade_this(), your goal is to write a series of tests around the student’s submission to determine whether or not it passes or fails. A passing or failing grade is signaled with pass() or fail(). You may provide a custom message or rely on the gradethis default passing or failing message.

In the case of our average exercise, a simple version of exercise grading might check that the student’s .result is equal to mean(1:10). If it is, the grading code returns a passing grade, otherwise a failing grade is returned.

```{r average-check}
grade_this({
  if (identical(.result, mean(1:10))) {
    pass("Great work! You calculated the average of the first ten integers.")
  }
  fail()
})
```

If a hypothetical student submitted mean(1:10) for this exercise, our checking code will return:

Great work! You calculated the average of the first ten integers.

If the student submits an incorrect answer, the default fail() message is encouraging and helpful:

Incorrect. I expected you to call mean() where you called min(). Give it another try.

This example highlights three important aspects of custom grading logic that we will cover in more detail:

1. grade_this() makes available a set of objects related to the exercise and the student’s submission, such as .result.

2. pass() and fail() signal a final grade as soon as they are called. There are also additional grade-signaling functions for common scenarios.

3. The default fail() message includes code feedback, if a solution is available. Code feedback, encouragement, and praise can be enabled or disabled for pass() and fail() grades directly or via global options.

Exercise Objects

The checking code you write inside grade_this() will be executed in an environment where a number of submission- and exercise-specific objects have been added.

Among these objects, grade_this() includes all of the objects provided by learnr to an exercise checking function. For convenience, gradethis also includes a few additional objects. To avoid name collisions with user or instructor code, the names of these objects all start with .:

Object	Description
.label	Label for exercise chunk
.solution_code	Code provided within the *-solution chunk for the exercise
.user_code	R code submitted by the user
.last_value	The last value from evaluating the user’s exercise submission
.result, .user	A direct copy of .last_value for friendlier naming
.solution	When accessed, will be the result of evaluating the .solution_code in a child environment of .envir_prep
.check_code	Code provided within the *-check (or *-code-check) chunk for the exercise
.envir_prep	A copy of the R environment before the execution of the chunk
.envir_result	The R environment after the execution of the chunk
.evaluate_result	The return value from the evaluate::evaluate function

You can reference any of these objects in your grading logic. Additionally, you can use debug_this() as your exercise checker or in lieu of a grade to return an informative message in the tutorial for your visual inspection of the value of each of these objects. This is useful when debugging or writing exercises.

Grading Helper Functions

There are a number of grading helper functions in addition to pass() and fail():

• pass_if_equal() and fail_if_equal() compare the submitted .result (or another object) to an expected value and pass or fail if the two values are equal.

• pass_if() and fail_if() pass or fail if their first argument, a condition, is TRUE.

• fail_if_code_feedback() fails if there are differences between the submitted code and the model solution code. This is useful when you want to use custom logic to check specific failure modes, but want to fall back to code comparison for modes you may not have anticipated.

Each of these functions signals a final grade as soon as they are called. This allows tutorial authors to construct grading logic in such a way that later tests presume earlier tests in the grading code were not triggered. If you are familiar with writing R functions, the pass() and fail() helper functions are similar to early return() statements.

Be careful to end your grade_this() grading code with a final fallback grade in case no other grades are returned. Typically, this fallback grade will be a call to pass() or fail() without any arguments.

Feedback Messages

The pass() and fail() functions all use the glue package for custom message templating. With a glue::glue() template string, you can easily interleave R values with the feedback.

fail(
  "Your code returned {round(.result, 2)}, but the average of `1:10` is 
  {if (.result > .solution) 'lower' else 'higher'} than that value."
)

Your code returned 5.5, but the average of 1:10 is higher than that value.

Notice that you may include R code wrapped in { } inside the template string and you may reference any of the exercise objects in that string.

There are a number of message components that you may want to consistently include in your feedback:

• pass() includes an additional praise argument to prepend random_praise() to the feedback

• fail() includes both hint and encouarge. When TRUE, hint adds code feedback to the failing message (if a solution is available) and encourage appends random_encouragement() to the end of the feedback message.

pass("That's exactly the average of 1 through 10.")

That's exactly the average of 1 through 10.

pass("That's exactly the average of 1 through 10.", praise = TRUE)

Magnificent! That's exactly the average of 1 through 10.

Each of the praise, encourage, and hint arguments can be enabled or disabled for all pass() or fail() grades via the global options set by gradethis_setup().

gradethis_setup(
  pass.praise = TRUE,
  fail.encourage = TRUE,
  fail.hint = TRUE
)

pass("That's exactly the average of 1 through 10.")

Magnificent! That's exactly the average of 1 through 10.

fail("Your answer was not the number I expected.")

Your answer was not the number I expected. I expected you to call mean() where you called min(). Don't give up now, try it one more time.

Notice that by globally turning on praise, encourage, and hint, custom feedback messages will contain the same components as the default pass and fail messages. (You can control those defaults with the pass and fail arguments of gradethis_setup().) Without those options enabled globally, custom messages will only contain the text included in the message.